The present invention relates to the post detection processing of a radar, and more particularly to a method of counting multiple targets moving in close proximity to one another.
Radars employing high resolution doppler processors are normally used for situations where there are a number of targets moving in close proximity to one another and an accurate measurement of range, angle or velocity is required for each target. However these radars are unable to obtain other information from the radar echo signals, such as the number of targets present in a target cluster, for example. Present radars are capable of identifying multiple targets which are spread out in the field of view thereof or are capable of searching for and tracking individual target clusters, without determining the number of targets in the tracked cluster. Generally, the task of counting multiple targets in a moving target cluster is left to the skill of a trained observer using a well designed display format of the post detection processed radar information. In a tactical military aircraft raid encounter, for example, the pilot must make an assessment of the target count with speed, accuracy and reliability in order to determine the form of action to take to protect himself and the equipment. Clearly, there is a need especially in this example for a method that can perform automatically and reliably the task of counting the number of targets in a raid cluster without burdening the pilot with an additional manual task during this critical period. Such a mechanism would be an invaluable aid to the pilot especially in the raid assessment mode.
A method capable of analyzing the doppler spectrum obtained from narrowband doppler filtering in the post detection processing of a radar for the purpose of counting the number of targets present appears, on the surface at least, somewhat simple. However, an accurate count of targets does not fall out naturally by simple examination of the narrowband doppler filter bank output because the doppler frequency signals are usually greatly perturbed by interference. Some interferring obstacles which any such target counting method would be expected to contend with are noise, amplitude scintillation, doppler frequency scintillation, and other signal perturbations which may be caused by echos from sources of the target aircraft other than skin returns, such as blades, turbines, motors and the like. Accordingly, the true nature of multitarget doppler frequency signatures can only be identified after an extensive study of many samples of recorded radar returns obtained under a variety of conditions. Even the way to deal with the doppler frequency filter signals and their perturbations resulting from controlled test environments is not uniquely nor immediately apparent.